Novel mechanism of Wnt signalling inhibition mediated by Dickkopf-1 interaction with LRP6/Arrow

Endocytic receptor-mediated control of morphogen signaling

Receptor-mediated endocytosis provides a mechanism by which cells take up signaling molecules from the extracellular space. Recent studies have shown that one class of endocytic receptors, the low-density lipoprotein receptor-related proteins (LRPs), is of particular relevance for embryonic development. In this Primer, we describe how LRPs constitute central pathways that modulate morphogen presentation to target tissues and cellular signal reception, and how LRP dysfunction leads to developmental disturbances in many species.

Pathogenic role of the Wnt signaling pathway activation in laser-induced choroidal neovascularization

PURPOSE

Choroidal neovascularization (CNV) is a severe complication of AMD. The Wnt signaling pathway has been shown to mediate angiogenesis. The purpose of this study was to investigate the pathogenic role of the Wnt pathway in CNV and explore the therapeutic potential of a novel Wnt signaling inhibitor in CNV.

METHODS

Adult rats and mice were photocoagulated using diode laser to induce CNV. On the same day, the animals were intravitreally injected with a monoclonal antibody (Mab2F1) blocking LRP6 or nonspecific mouse IgG. The Wnt signaling activation and target gene expression in the eyecup were determined by Western blot analysis. Fundus angiography was used to examine leakage from the laser lesion. CNV areas were measured on choroidal flatmount using FITC-dextran.

RESULTS

Levels of Wnt pathway components and Wnt target gene expression were elevated in both laser-induced CNV rat and mouse eyecups, suggesting activation of the Wnt pathway. Significant suppression of Wnt signaling was observed in the Mab2F1 treatment group. Mab2F1 decreased vascular leakage from CNV lesions and reduced the neovascular area in laser-induced CNV rats. Mab2F1 inhibited the hypoxia-induced activation of Wnt signaling in cultured RPE cells. Mab2F1 also ameliorated retinal inflammation and vascular leakage in the eyecups of very low-density lipoprotein receptor knockout mice, a model of subretinal neovascularization.

CONCLUSIONS

The Wnt pathway is activated in the laser-induced CNV models and plays a pathogenic role in CNV. Blockade of Wnt signaling using an anti-LRP6 antibody has therapeutic potential in CNV.

Physiological and molecular determinants of embryo implantation

Embryo implantation involves the intimate interaction between an implantation-competent blastocyst and a receptive uterus, which occurs in a limited time period known as the window of implantation. Emerging evidence shows that defects originating during embryo implantation induce ripple effects with adverse consequences on later gestation events, highlighting the significance of this event for pregnancy success. Although a multitude of cellular events and molecular pathways involved in embryo-uterine crosstalk during implantation have been identified through gene expression studies and genetically engineered mouse models, a comprehensive understanding of the nature of embryo implantation is still missing. This review focuses on recent progress with particular attention to physiological and molecular determinants of blastocyst activation, uterine receptivity, blastocyst attachment and uterine decidualization. A better understanding of underlying mechanisms governing embryo implantation should generate new strategies to rectify implantation failure and improve pregnancy rates in women.

Clinico-epigenetic combination including quantitative methylation value of DKK3 augments survival prediction of the patient with cervical cancer

PURPOSE

DKK3 is a target of methylation in various cancers and has been studied by a non-quantitative method. We assessed the quantitative methylation levels of DKK3 in cervical carcinoma, determined the potential clinical correlations, and tested whether the combination of clinical and epigenetic factors augmented the prediction power of prognosis.

METHODS

Sixty-two patients with cervical squamous cell carcinoma were included in this study. Quantitative methylation levels were evaluated by pyrosequencing. Clinical and pathologic findings were obtained from medical records. Survival data were analyzed using Kaplan-Meier estimates and compared with the log-rank test. The best clinico-epigenetic combinations were found using the Cox proportional hazard model.

RESULTS

Four of five CpG positions of DKK3 were strongly methylated in cervical carcinoma compared to normal controls (p = 0.0048). The methylation in positions 1 and/or 2 were stronger in patients with higher serum levels of the SCC tumor marker and/or larger tumors (p = 0.01). The patients with a methylation level ≥26.3 % at position 1 had a lower survival rate than the patients with methylation levels at position 1 that were <26.3 % (p = 0.03). The combination of methylation level of position 1, position 3, age, parametrial invasion, and lymphovascular space invasion (LVSI) have a significant correlation with survival (p = 0.0006). Recurrence was significantly related to the combination of methylation level of position 2, position 3, age, parametrium, and LVSI (p = 0.0041).

CONCLUSIONS

DKK3 methylation is unfavorable to prognosis. This study defined a threshold level of methylation associated with recurrence-free survival and, furthermore, identified novel clinico-epigenetic combinations predicting disease survival or recurrence.

Strategies for anti-fibrotic therapies

The fibrotic diseases encompass a wide spectrum of entities including such multisystemic diseases as systemic sclerosis, nephrogenic systemic fibrosis and sclerodermatous graft versus host disease, as well as organ-specific disorders such as pulmonary, liver, and kidney fibrosis. Collectively, given the wide variety of affected organs, the chronic nature of the fibrotic processes, and the large number of individuals suffering their devastating effects, these diseases pose one of the most serious health problems in current medicine and a serious economic burden to society. Despite these considerations there is currently no accepted effective treatment. However, remarkable progress has been achieved in the elucidation of their pathogenesis including the identification of the critical role of myofibroblasts and the determination of molecular mechanisms that result in the transcriptional activation of the genes responsible for the fibrotic process. Here we review the origin of the myofibroblast and discuss the crucial regulatory pathways involving multiple growth factors and cytokines that participate in the pathogenesis of the fibrotic process. Potentially effective therapeutic strategies based upon this new information are considered in detail and the major challenges that remain and their possible solutions are presented. It is expected that translational efforts devoted to convert this new knowledge into novel and effective anti-fibrotic drugs will be forthcoming in the near future. This article is part of a Special Issue entitled: Fibrosis: Translation of basic research to human disease.

Estrogen regulation of Dkk1 and Wnt/β-Catenin signaling in neurodegenerative disease

17β-estradiol (E2 or estrogen) is an endogenous steroid hormone that is well known to exert neuroprotection. Along these lines, one mechanism through which E2 protects the hippocampus from cerebral ischemia is by preventing the post-ischemic elevation of Dkk1, a neurodegenerative factor that serves as an antagonist of the canonical Wnt signaling pathway, and simultaneously inducing pro-survival Wnt/β-Catenin signaling in hippocampal neurons. Intriguingly, while expression of Dkk1 is required for proper neural development, overexpression of Dkk1 is characteristic of many neurodegenerative diseases, such as stroke, Alzheimer's disease, Parkinson's disease, and temporal lobe epilepsy. In this review, we will briefly summarize the canonical Wnt signaling pathway, highlight the current literature linking alterations of Dkk1 and Wnt/β-Catenin signaling with neurological disease, and discuss E2's role in maintaining the delicate balance of Dkk1 and Wnt/β-Catenin signaling in the adult brain. Finally, we will consider the implications of long-term E2 deprivation and hormone therapy on this crucial neural pathway. This article is part of a Special Issue entitled Hormone Therapy.

High serum sclerostin predicts the occurrence of osteoporotic fractures in postmenopausal women: the Center of Excellence for Osteoporosis Research Study

Sclerostin regulates bone formation by inhibiting Wnt pathway signaling. Low circulating sclerostin levels cause high bone mass. We hypothesized that postmenopausal women with increased sclerostin levels have a greater risk for osteoporosis-related fractures. We examined the association between circulating sclerostin together with bone turnover markers and osteoporosis-related fracture risk in 707 postmenopausal women, in a population-based study with a mean follow-up period of 5.2 ± 1.3 years. Multivariate Cox proportional hazards regression models were used to analyze fracture risk, adjusted for age, body mass index, and other confounding risk factors. High sclerostin levels were strongly associated with increased fracture risk. After adjustment for age and other confounders, the relative fracture risk was more than sevenfold among postmenopausal women for each 1-SD increment increase in sclerostin level. Women in the highest quartile of sclerostin levels had about a 15-fold increase in fracture risk. Results were similar when we compared sclerostin at the 1-year visit to an average of two to three annual measurements. Fracture risk attributable to sclerostin levels was 56.6% in the highest quartile. Only high levels of bone resorption markers (plasma cross-linked C-terminal telopeptide of type 1 collagen [p-CTx], urinary CTx [u-CTx], and urinary N-telopeptide of type 1 collagen [u-NTx]) were predictive of osteoporosis-related fractures but at much lower hazard ratio (HR) values than that of serum sclerostin. Associations between sclerostin levels and fracture risk were independent of bone mineral density and other confounding risk factors. High sclerostin levels are a strong and independent risk factor for osteoporosis-related fractures among postmenopausal women. © 2012 American Society for Bone and Mineral Research.

Differential requirements for Wnt and Notch signaling in hematopoietic versus thymic niches

All blood cells are derived from multipotent stem cells, the so-called hematopoietic stem cells (HSCs), that in adults reside in the bone marrow. Most types of blood cells also develop there, with the notable exception of T lymphocytes that develop in the thymus. For both HSCs and developing T cells, interactions with the surrounding microenvironment are critical in regulating maintenance, differentiation, apoptosis, and proliferation. Such specialized regulatory microenvironments are referred to as niches and provide both soluble factors as well as cell-cell interactions between niche component cells and blood cells. Two pathways that are critical for early T cell development in the thymic niche are Wnt and Notch signaling. These signals also play important but controversial roles in the HSC niche. Here, we review the differences and similarities between the thymic and hematopoietic niches, with particular focus on Wnt and Notch signals, as well as the latest insights into regulation of these developmentally important pathways.

Using components of the vitamin D pathway to prevent and treat colon cancer

The objective of this review was to analyze the components of vitamin D and their potential usefulness in preventing and treating colorectal cancer. The active form of vitamin D, 1α,25(OH(2) )D(3) , targets the wnt/β-catenin pathway by upregulating key tumor suppressor genes such as E-cadherin, which promotes an epithelial phenotype, but this is only possible when the vitamin D receptor (VDR) is present. Colorectal cell lines have shown that VDR expression levels decrease in the later stages of colon cancer. In colorectal cancers with low VDR expression, treatments to increase VDR expression could target alterations at the genomic and epigenomic levels by modulating transcription factors such as SNAIL1 and by utilizing histone deacetyltransferase inhibitors, respectively. Finally, epidemiological studies suggest that the current US Recommended Dietary Allowance should be increased to 2,000 IU in order to raise serum 25(OH)D(3) levels above 30 ng/mL. This increase in vitamin D status can be obtained most efficiently from sun exposure or vitamin D supplementation. In summary, vitamin D and its metabolites could be utilized in strategies to treat and prevent colon cancer.

Cutaneous wound healing: recruiting developmental pathways for regeneration

Following a skin injury, the damaged tissue is repaired through the coordinated biological actions that constitute the cutaneous healing response. In mammals, repaired skin is not identical to intact uninjured skin, however, and this disparity may be caused by differences in the mechanisms that regulate postnatal cutaneous wound repair compared to embryonic skin development. Improving our understanding of the molecular pathways that are involved in these processes is essential to generate new therapies for wound healing complications. Here we focus on the roles of several key developmental signaling pathways (Wnt/β-catenin, TGF-β, Hedgehog, Notch) in mammalian cutaneous wound repair, and compare this to their function in skin development. We discuss the varying responses to cutaneous injury across the taxa, ranging from complete regeneration to scar tissue formation. Finally, we outline how research into the role of developmental pathways during skin repair has contributed to current wound therapies, and holds potential for the development of more effective treatments.

Wnt signaling in breast organogenesis

Wnt signals play a critical role in regulating the normal development of the mammary gland and dysregulation of Wnt signaling causes breast cancer. This pathway is involved in the earliest development of the mammary gland in embryos and its role extends through the functional differentiation of the gland during pregnancy. In this review, we summarize the molecular mechanisms through which Wnts regulate mammary gland development in the mouse.

In vascular smooth muscle cells paricalcitol prevents phosphate-induced Wnt/β-catenin activation

The present study investigates the differential effect of two vitamin D receptor agonists, calcitriol and paricalcitol, on human aortic smooth muscle cells calcification in vitro. Human vascular smooth muscle cells were incubated in a high phosphate (HP) medium alone or supplemented with either calcitriol 10(-8)M (HP + CTR) or paricalcitol 3·10(-8) M (HP + PC). HP medium induced calcification, which was associated with the upregulation of mRNA expression of osteogenic factors such as bone morphogenetic protein 2 (BMP2), Runx2/Cbfa1, Msx2, and osteocalcin. In these cells, activation of Wnt/β-catenin signaling was evidenced by the translocation of β-catenin into the nucleus and the increase in the expression of direct target genes as cyclin D1, axin 2, and VCAN/versican. Addition of calcitriol to HP medium (HP + CTR) further increased calcification and also enhanced the expression of osteogenic factors together with a significant elevation of nuclear β-catenin levels and the expression of cyclin D1, axin 2, and VCAN. By contrast, the addition of paricalcitol (HP + PC) not only reduced calcification but also downregulated the expression of BMP2 and other osteoblastic phenotype markers as well as the levels of nuclear β-catenin and the expression of its target genes. The role of Wnt/β-catenin on phosphate- and calcitriol-induced calcification was further demonstrated by the inhibition of calcification after addition of Dickkopf-related protein 1 (DKK-1), a specific natural antagonist of the Wnt/β-catenin signaling pathway. In conclusion, the differential effect of calcitriol and paricalcitol on vascular calcification appears to be mediated by a distinct regulation of the BMP and Wnt/β-catenin signaling pathways.

Genetic control of ovarian development

During embryonic development, ovarian somatic cells embark on a course that is separate from male somatic cells and from indifferent precursor cells. While the former aspect of ovarian development is well known, the latter has not received much attention until recently. This review attempts to integrate the most recent work regarding the differentiation of ovarian somatic cells. The discussion of the parallel development of the testis is limited to the key differences only. Similarly, germ cell development will be introduced only inasmuch as it becomes necessary to draw attention to a particular aspect of the somatic component differentiation. Finally, while postnatal ovarian development and folliculogenesis undoubtedly provide the ultimate morphological and functional fitness tests for the ovarian somatic cells, postnatal phenotypes will be only referred to when they have already been connected to genes that are expressed during embryogenesis.

Genistein affects histone modifications on Dickkopf-related protein 1 (DKK1) gene in SW480 human colon cancer cell line

Genistein (GEN) is a plant-derived isoflavone and can block uncontrolled cell growth in colon cancer by inhibiting the WNT signaling pathway. This study aimed to test the hypothesis that the enhanced gene expression of the WNT signaling pathway antagonist, DKK1 by genistein treatment is associated with epigenetic modifications of the gene in colon cancer cells. Genistein treatment induced a concentration-dependent G2 phase arrest in the human colon cancer cell line SW480 and reduced cell proliferation. Results from several other human colon cancer cell lines confirmed the growth inhibitory effects of genistein. Overexpression of DKK1 confirmed its involvement in growth inhibition. Knockdown of DKK1 expression by siRNA slightly induced cell growth. DKK1 gene expression was increased by genistein in SW480 and HCT15 cells. DNA methylation at the DKK1 promoter was not affected by genistein treatment in all the cell lines tested. On the other hand, genistein induced histone H3 acetylation of the DKK1 promoter region in SW480 and HCT15 cells. This indicates that increased histone acetylation is associated with the genistein-induced DKK1 expression. The association between histone acetylation and DKK1 gene expression is confirmed by the histone deacetylase inhibitor trichostatin A (TSA) treatment. In conclusion, genistein treatment decreases cell growth and proliferation in colon cancer cell lines. The effect is associated with the increased DKK1 expression through the induction of histone acetylation at the DKK1 promoter region.

Wnt signaling enhances neurogenesis and improves neurological function after focal ischemic injury

Stroke potently stimulates cell proliferation in the subventricular zone of the lateral ventricles with subsequent neuroblast migration to the injured striatum and cortex. However, most of the cells do not survive and mature. Extracellular Wnt proteins promote adult neurogenesis in the neurogenic niches. The aim of the study was to examine the efficacy of Wnt signaling on neurogenesis and functional outcome after focal ischemic injury. Lentivirus expressing Wnt3a-HA (LV-Wnt3a-HA) or GFP (LV-GFP) was injected into the striatum or subventricular zone of mice. Five days later, focal ischemic injury was induced by injection of the vasoconstrictor endothelin-1 into the striatum of the same hemisphere. Treatment with LV-Wnt3a-HA into the striatum significantly enhanced functional recovery after ischemic injury and increased the number of BrdU-positive cells that differentiated into mature neurons in the ischemic striatum by day 28. Treatment with LV-Wnt3a-HA into the subventricular zone significantly enhanced functional recovery from the second day after injury and increased the number of immature neurons in the striatum and subventricular zone. This was accompanied by reduced dissemination of the neuronal injury. Our data indicate that Wnt signaling appears to contribute to functional recovery after ischemic injury by increasing neurogenesis or neuronal survival in the striatum.

BAC-Dkk3-EGFP transgenic mouse: an in vivo analytical tool for Dkk3 expression

Dickkopf (DKK) family proteins are secreted modulators of the Wnt signaling pathway and are capable of regulating the development of many organs and tissues. We previously identified Dkk3 to be a molecule predominantly expressed in the mouse embryonic retina. However, which cell expresses Dkk3 in the developing and mature mouse retina remains to be elucidated. To examine the precise expression of the Dkk3 protein, we generated BAC-Dkk3-EGFP transgenic mice that express EGFP integrated into the Dkk3 gene in a BAC plasmid. Expression analysis using the BAC-Dkk3-EGFP transgenic mice revealed that Dkk3 is expressed in retinal progenitor cells (RPCs) at embryonic stages and in Müller glial cells in the adult retina. Since Müller glial cells may play a potential role in retinal regeneration, BAC-Dkk3-EGFP mice could be useful for retinal regeneration studies.

The influence of fluoride on the expression of inhibitors of Wnt/β-catenin signaling pathway in rat skin fibroblast Cells

The effective therapy of fluoride-induced bone diseases requires an understanding of the mechanism of the disorders. Changes in the inhibitors of the Wnt/β-catenin pathway, Dickkopf-1 (Dkk-1) and Sclerostin (SOST),were studied in supernatants harvested from rat skin fibroblasts cultured with varied doses of fluoride. The contents of SOST and Dkk-1 in fibroblast supernatants were assessed at four exposure time-points and investigated by using the method of ELISA. Compared to the relevant controls(0 mg F(−)/L), a significant decrease of the concentrations of SOST and Dkk-1 was observed as the fluoride concentration increased. Compared to the relevant time controls (24 h), a significant decrease of the concentrations of SOST and Dkk-1 was observed with the extension of time. Our results suggest that the Wnt/β-catenin pathway inhibitors Dkk-1 and SOST play an important role in skeletal fluorosis. They can be used as important indications for diagnosing bone metabolism changes caused by fluoride exposure and therapeutic targets in diseases resulting from fluoride exposure.

Chemical and genetic evidence for the involvement of Wnt antagonist Dickkopf2 in regulation of glucose metabolism

Mutations in Wnt receptor LRP5/6 and polymorphism in Wnt-regulated transcription factor TCF7L2 are associated with dysregulation of glucose metabolism. However, it is not clear whether Wnt antagonist Dickkopf (Dkk) has a significant role in the regulation of glucose metabolism. Here, we identified small-molecule inhibitors of Wnt antagonist Dkk through molecular modeling, computation-based virtual screens, and biological assays. One of the Dkk inhibitors reduced basal blood-glucose concentrations and improved glucose tolerance in mice. This Dkk inhibitor appeared to act through DKK2 because the inhibitor exerted no additional effects on glucose metabolism in the Dkk2(-/-) mice. Our study of Dkk2(-/-) mice showed that DKK2 deficiency was associated with increased hepatic glycogen accumulation and decreased hepatic glucose output. DKK2 deficiency did not cause in increase in insulin production but resulted in increased Wnt activity and GLP1 production in the intestines. Given that the Dkk inhibitor improved glucose tolerance in a murine model of type 2 diabetes (db/db), we suggest that DKK2 may be a potential therapeutic target for treating type 2 diabetes.

Three decades of Wnts: a personal perspective on how a scientific field developed

Wnt genes and components of Wnt signalling pathways have been implicated in a wide spectrum of important biological phenomena, ranging from early organismal development to cell behaviours to several diseases, especially cancers. Emergence of the field of Wnt signalling can be largely traced back to the discovery of the first mammalian Wnt gene in 1982. In this essay, we mark the thirtieth anniversary of that discovery by describing some of the critical scientific developments that led to the flowering of this field of research.

LGR6 is a high affinity receptor of R-spondins and potentially functions as a tumor suppressor

BACKGROUND

LGR6 (leucine-rich repeat containing, G protein-coupled receptor 6) is a member of the rhodopsin-like seven transmembrane domain receptor superfamily with the highest homology to LGR4 and LGR5. LGR6 was found as one of the novel genes mutated in colon cancer through total exon sequencing and its promoter region is hypermethylated in 20-50% of colon cancer cases. In the skin, LGR6 marks a population of stem cells that can give rise to all cell lineages. Recently, we and others demonstrated that LGR4 and LGR5 function as receptors of R-spondins to potentiate Wnt/β-catenin signaling. However, the binding affinity and functional response of LGR6 to R-spondins, and the activity of colon cancer mutants of LGR6 have not been determined.

PRINCIPAL FINDINGS

We found that LGR6 also binds and responds to R-spondins 1-3 with high affinity to enhance Wnt/β-catenin signaling through increased LRP6 phosphorylation. Similar to LGR4 and LGR5, LGR6 is not coupled to heterotrimeric G proteins or to β-arrestin following R-spondin stimulation. Functional and expression analysis of three somatic mutations identified in colon cancer samples indicates that one mutant fails to bind and respond to R-spondin (loss-of-function), but the other two have no significant effect on receptor function. Overexpression of wild-type LGR6 in HeLa cells leads to increased cell migration following co-treatment with R-spondin1 and Wnt3a when compared to vector control cells or cells overexpressing the loss-of-function mutant.

CONCLUSIONS

LGR6 is a high affinity receptor for R-spondins 1-3 and potentially functions as a tumor suppressor despite its positive effect on Wnt/β-catenin signaling.

The structural basis of DKK-mediated inhibition of Wnt/LRP signaling

Low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/6) mediate canonical Wnt-β-catenin signaling by forming a complex with the co-receptor Frizzled, which binds to Wnt proteins. Dickkopf (DKK)-related proteins inhibit the Wnt signaling pathway by directly binding to the ectodomains of LRP5/6. However, the mechanism for DKK-mediated antagonism has not been fully understood as of yet. Crystal structures of the LRP6 ectodomain in complex with DKK1, along with mutagenesis studies, provide considerable insights into the molecular basis for DKK-mediated inhibition and Wnt signaling through LRP5/6.

A new validated mathematical model of the Wnt signalling pathway predicts effective combinational therapy by sFRP and Dkk

The Wnt signalling pathway controls cell proliferation and differentiation, and its deregulation is implicated in different diseases including cancer. Learning how to manipulate this pathway could substantially contribute to the development of therapies. We developed a mathematical model describing the initial sequence of events in the Wnt pathway, from ligand binding to β-catenin accumulation, and the effects of inhibitors, such as sFRPs (secreted Frizzled-related proteins) and Dkk (Dickkopf). Model parameters were retrieved from experimental data reported previously. The model was retrospectively validated by accurately predicting the effects of Wnt3a and sFRP1 on β-catenin levels in two independent published experiments (R2 between 0.63 and 0.91). Prospective validation was obtained by testing the model's accuracy in predicting the effect of Dkk1 on Wnt-induced β-catenin accumulation (R2≈0.94). Model simulations under different combinations of sFRP1 and Dkk1 predicted a clear synergistic effect of these two inhibitors on β-catenin accumulation, which may point towards a new treatment avenue. Our model allows precise calculation of the effect of inhibitors applied alone or in combination, and provides a flexible framework for identifying potential targets for intervention in the Wnt signalling pathway.

Colon cancer stem cells--from basic to clinical application

Based on cancer stem cell (CSC) concept of carcinogenesis, tumors represent complex heterogeneous organ-like systems with a hierarchical cellular organization, and only minority phenotypic subpopulations with stem-like properties possess a dual ability to self-renew indefinitely and produce all the heterogeneous cell phenotypes comprising the bulk tumor cells. Large experimental and clinical data indicate that conventional anti-cancer therapies cannot eradicate CSCs, and moreover, they usually increase their number leading to cancer recurrence and further drug resistance. In this review, several current controversies in the CSC field and recent studies, which help to shed light on their origin, are discussed. The emerging necessity for the development of complex, multimodal CSC-targeted treatment strategies, which combine conventional therapeutics with promising pathway-specific modulators, and natural compounds, which can improve the efficacy of conventional anti-cancer therapeutics and decrease their undesirable side effects is presented. Also, novel requirements and criteria necessary for evaluation of the CSC-targeted drug efficacy and relevant experimental models are discussed.

sizzled function and secreted factor network dynamics

Studies on the role of the E-box binding transcription factor Snail2 (Slug) in the induction of neural crest by mesoderm (Shi et al., 2011) revealed an unexpected increase in the level of sizzled RNA in the dorsolateral mesodermal zone (DMLZ) of morphant Xenopus embryos. sizzled encodes a secreted protein with both Wnt and BMP inhibitor activities. Morpholino-mediated down-regulation of sizzled expression in one cell of two cell embryos or the C2/C3 blastomeres of 32-cell embryos, which give rise to the DLMZ, revealed decreased expression of the mesodermal marker brachyury and subsequent defects in neural crest induction, pronephros formation, and muscle patterning. Loss of sizzled expression led to decreases in RNAs encoding the secreted Wnt inhibitor SFRP2 and the secreted BMP inhibitor Noggin; the sizzled morphant phenotype could be rescued by co-injection of RNAs encoding Noggin and either SFRP2 or Dickkopf (a mechanistically distinct Wnt inhibitor). Together, these observations reveal that sizzled, in addition to its established role in dorsal-ventral patterning, is also part of a dynamic BMP and Wnt signaling network involved in both mesodermal patterning and neural crest induction.

Activation of canonical Wnt signalling is required for TGF-β-mediated fibrosis

The transforming growth factor-β (TGF-β) signalling pathway is a key mediator of fibroblast activation that drives the aberrant synthesis of extracellular matrix in fibrotic diseases. Here we demonstrate a novel link between transforming growth factor-β and the canonical Wnt pathway. TGF-β stimulates canonical Wnt signalling in a p38-dependent manner by decreasing the expression of the Wnt antagonist Dickkopf-1. Tissue samples from human fibrotic diseases show enhanced expression of Wnt proteins and decreased expression of Dickkopf-1. Activation of the canonical Wnt pathway stimulates fibroblasts in vitro and induces fibrosis in vivo. Transgenic overexpression of Dickkopf-1 ameliorates skin fibrosis induced by constitutively active TGF-β receptor type I signalling and also prevents fibrosis in other TGF-β-dependent animal models. These findings demonstrate that canonical Wnt signalling is necessary for TGF-β-mediated fibrosis and highlight a key role for the interaction of both pathways in the pathogenesis of fibrotic diseases.

Aberrant activation of the TGF-β pathway leads to fibrotic disease. Distler and colleagues show that TGF-β-mediated fibrosis requires the decrease of Dickkopf-1, an antagonist of canonical Wnt signalling, suggesting that the two pathways interact for the manifestation of this disease.

NLK positively regulates Wnt/β-catenin signalling by phosphorylating LEF1 in neural progenitor cells

Nemo-like kinase (NLK/Nlk) is an evolutionarily conserved protein kinase involved in Wnt/β-catenin signalling. However, the roles of NLK in Wnt/β-catenin signalling in vertebrates remain unclear. Here, we show that inhibition of Nlk2 function in zebrafish results in decreased Lymphoid enhancer factor-1 (Lef1)-mediated gene expression and cell proliferation in the presumptive midbrain, resulting in a reduction of midbrain tectum size. These defects are related to phosphorylation of Lef1 by Nlk2. Thus, Nlk2 is essential for the phosphorylation and activation of Lef1 transcriptional activity in neural progenitor cells (NPCs). In NPC-like mammalian cells, NLK is also required for the phosphorylation and activation of LEF1 transcriptional activity. Phosphorylation of LEF1 induces its dissociation from histone deacetylase, thereby allowing transcription activation. Furthermore, we demonstrate that NLK functions downstream of Dishevelled (Dvl) in the Wnt/β-catenin signalling pathway. Our findings reveal a novel role of NLK in the activation of the Wnt/β-catenin signalling pathway.

Integrin α9β1 promotes malignant tumor growth and metastasis by potentiating epithelial-mesenchymal transition

The integrin α9β1 binds a number of extracellular matrix components to mediate cell adhesion, migration and tissue invasion. Although expressed in a variety of normal human cells including endothelium, it is also expressed in cancer cells. We have previously shown that α9β1 binds VEGF-A to facilitate angiogenesis, an important component of the tumor microenvironment. As α9β1 induces accelerated cancer cell migration, we wished to determine what role it played in cancer growth and metastasis. In this study, we show that α9β1 expression induces molecular changes consistent with epithelial-mesenchymal transition. In addition, we found that α9β1 forms a tri-partite protein complex with β-catenin and E-cadherin, which dissociates following integrin activation and subsequent src and β-catenin phosphorylation. These findings were consistent in cells in which: α9β1 was exogenously over-expressed, or when its expression was suppressed in cancer cells endogenously expressing α9β1. These in vitro results are biologically significant as α9β1-expressing cancer cells induce greater tumor growth and metastases in mice as compared to the cells without α9β1 expression or when integrin expression is suppressed. Furthermore, integrin α9β1 is expressed in primary human small cell lung cancer and patients having a high expression of α9β1 demonstrated significantly worse long-term survival compared with patients with low α9β1 expression. These findings highlight a novel mechanism of integrin α9β1 function in human cancer.

New insights into the mechanism of Wnt signaling pathway activation

Wnts compromise a large family of secreted, hydrophobic glycoproteins that control a variety of developmental and adult processes in all metazoan organisms. Recent advances in the Wnt-signal studies have revealed that distinct Wnts activate multiple intracellular cascades that regulate cellular proliferation, differentiation, migration, and polarity. Although the mechanism by which Wnts regulate different pathways selectively remains to be clarified, evidence has accumulated that in addition to the formation of ligand-receptor pairs, phosphorylation of receptors, receptor-mediated endocytosis, acidification, and the presence of cofactors, such as heparan sulfate proteoglycans, are also involved in the activation of specific Wnt pathways. Here, we review the mechanism of activation in Wnt signaling initiated on the cell-surface membrane. In addition, the mechanisms for fine-tuning by cross talk between Wnt and other signaling are also discussed.

Dickkopf 1 promotes regression of hair follicles

Recently, we suggested that Dickkopf 1 (DKK-1) is a pathogenic mediator involved in male pattern baldness. As premature catagen onset is a key characteristic of male pattern baldness, in this study, we evaluated whether DKK-1 has a role as a catagen inducer in hair cycling. Herein, we report that recombinant human DKK-1 (rhDKK-1) injection into the hypodermis of mice during anagen caused premature onset of catagen, whereas neutralizing DKK-1 antibody delayed anagen-to-catagen transition in mice. Moreover, treatment with rhDKK-1 led to a decrease in final hair follicle length, whereas DKK-1 antibody led to an increase compared with control animals. In addition, DKK-1 and DKK-1 messenger RNA expression is most upregulated in follicular keratinocytes of late anagen in depilation-induced hair cycle progression. Moreover, we observed that rhDKK-1 blocks canonical Wnt-mediated activation of β-catenin signaling and induces the proapoptotic protein Bax, resulting in apoptosis in outer root sheath keratinocytes. Taken together, our data strongly suggest that DKK-1 is involved in anagen-to-catagen transition in the hair cycle by regulating the activity of follicular keratinocytes.

Wnt signaling pathways in urological cancers: past decades and still growing

The Wnt signaling pathway is involved in a wide range of embryonic patterning events and maintenance of homeostasis in adult tissues. The pathological role of the Wnt pathway has emerged from studies showing a high frequency of specific human cancers associated with mutations that constitutively activate the transcriptional response of these pathways. Constitutive activation of the Wnt signaling pathway is a common feature of solid tumors and contributes to tumor development, progression and metastasis in various cancers. In this review, the Wnt pathway will be covered from the perspective of urological cancers with emphasis placed on the recent published literature. Regulation of the Wnt signaling pathway by microRNAs (miRNA), small RNA sequences that modify gene expression profiles will also be discussed. An improved understanding of the basic genetics and biology of Wnt signaling pathway will provide insights into the development of novel chemopreventive and therapeutic strategies for urological cancers.

Localization of glypican-4 in different membrane microdomains is involved in the regulation of Wnt signaling

Glypicans are members of the heparan sulfate proteoglycans (HSPGs) and are involved in various growth factor signaling mechanisms. Although HSPGs affect the β-catenin-dependent and -independent pathways of Wnt signaling, how they regulate distinct Wnt pathways is not clear. It has been suggested that the β-catenin-dependent pathway is initiated through receptor endocytosis in lipid raft microdomains and the independent pathway is activated through receptor endocytosis in non-lipid raft microdomains. Here, evidence is presented that glypican-4 (GPC4) is localized to both membrane microdomains and that the localization affects its ability to regulate distinct Wnt pathways. GPC4 bound to Wnt3a and Wnt5a, which activate the β-catenin-dependent and -independent pathways, respectively, and colocalized with Wnts on the cell surface. LRP6, one of Wnt3a coreceptors, was present in lipid raft microdomains, whereas Ror2, one of Wnt5a coreceptors, was localized to non-lipid raft microdomains. Expression of GPC4 enhanced the Wnt3a-dependent β-catenin pathway and the Wnt5a-dependent β-catenin-independent pathway, and knockdown of GPC4 suppressed both pathways. A GPC4 mutant that was localized to only non-lipid raft microdomains inhibited the β-catenin-dependent pathway but enhanced the β-catenin-independent pathway. These results suggest that GPC4 concentrates Wnt3a and Wnt5a to the vicinity of their specific receptors in different membrane microdomains, thereby regulating distinct Wnt signaling.

DNA methylation status of REIC/Dkk-3 gene in human malignancies

Purpose

The REIC (reduced expression in immortalized cells)/Dkk-3 is down-regulated in various cancers and considered to be a tumor suppressor gene. REIC/Dkk-3 mRNA has two isoforms (type-a,b). REIC type-a mRNA has shown to be a major transcript in various cancer cells, and its promoter activity was much stronger than that of type-b. In this study, we examined the methylation status of REIC/Dkk-3 type-a in a broad range of human malignancies.

Methods

We examined REIC/Dkk-3 type-a methylation in breast cancers, non-small-cell lung cancers, gastric cancers, colorectal cancers, and malignant pleural mesotheliomas using a quantitative combined bisulfite restriction analysis assay and bisulfate sequencing. REIC/Dkk-3 type-a and type-b expression was examined using reverse transcriptional PCR. The relationships between the methylation and clinicopathological factors were analyzed.

Results

The rate of REIC/Dkk-3 type-a methylation ranged from 26.2 to 50.0% in the various primary tumors that were examined. REIC/Dkk-3 type-a methylation in breast cancer cells was significantly heavier than that in the other cell lines that we tested. REIC/Dkk-3 type-a methylation was inversely correlated with REIC/Dkk-3 type-a expression. There was a correlation between REIC/Dkk-3 type-a and type-b mRNA expression. REIC/Dkk-3 type-a expression was restored in MDA-MB-231 cells using 5-aza-2′-deoxycytidine treatment. We found that estrogen receptor–positive breast cancers were significantly more common among the methylated group than among the non-methylated group.

Conclusions

REIC/Dkk-3 type-a methylation was frequently detected in a broad range of cancers and appeared to play a key role in silencing REIC/Dkk-3 type-a expression in these malignancies.

Functional Characterization of a Small-Molecule Inhibitor of the DKK1-LRP6 Interaction

Background. DKK1 antagonizes canonical Wnt signalling through high-affinity binding to LRP5/6, an essential component of the Wnt receptor complex responsible for mediating downstream canonical Wnt signalling. DKK1 overexpression is known for its pathological implications in osteoporosis, cancer, and neurodegeneration, suggesting the interaction with LRP5/6 as a potential therapeutic target. Results. We show that the small-molecule NCI8642 can efficiently displace DKK1 from LRP6 and block DKK1 inhibitory activity on canonical Wnt signalling, as shown in binding and cellular assays, respectively. We further characterize NCI8642 binding activity on LRP6 by Surface Plasmon Resonance (SPR) technology. Conclusions. This study demonstrates that the DKK1-LRP6 interaction can be the target of small molecules and unlocks the possibility of new therapeutic tools for diseases associated with DKK1 dysregulation.

Antagonists of LRP6 regulate PTH-induced cAMP generation

LRP6 is a common coreceoptor for different G protein-coupled seven-transmembrane receptors in production of cAMP. Extracelluar proteins sclerostin and DKK1, initially identified as antagonists for Wnt signaling by binding to LRP6, are negative regulators for bone formation. Here, we show that both sclerostin and DKK1 inhibit PTH-stimulated cAMP production. In addition, PTH suppresses expression of sclerostin in osteocytes in mice. We also found that sclerostin and DKK1 binds to LRP6 as antagonists to increase the availability of LRP6 to facilitate PTH signaling in a positive-feedback fashion. These studies reveal a previously unrecognized function of sclerostin and DKK1, which provides an alternative explanation for the application of sclerostin and DKK1 neutralization on enhancing bone formation as a potential therapy for skeletal diseases.

The role of WNT10B in physiology and disease

Wnt10b is a member of the Wnt ligand gene family that encodes for secreted proteins, which activate the ancient and highly conserved Wnt signalling cascade. The Wnt pathway has been shown to be essential for embryonic development, tissue integrity, and stem cell activity, but if deregulated, also causes disease such as cancer. Although the 19 different Wnt ligands found in both human and mouse can activate several branches of the Wnt pathway, WNT10B specifically activates canonical Wnt/β-catenin signalling and thus triggers β-catenin/LEF/TCF-mediated transcriptional programs. In this review, we highlight the unique functions of WNT10B and mechanisms of how WNT10B acts in the immune system, mammary gland, adipose tissue, bone and skin. In these organs, WNT10B has been well established to be involved in signalling networks controlling stemness, pluripotency and cell fate decisions. Deregulation of these processes causes diseases such as breast cancer, obesity and osteoporosis. Compelling evidence suggests that WNT10B is a valuable candidate for the development of therapeutic regimens for human diseases.

Reversible cell-cycle entry in adult kidney podocytes through regulated control of telomerase and Wnt signaling

Mechanisms of epithelial cell renewal remain poorly understood in the mammalian kidney, particularly in the glomerulus, a site of cellular damage in chronic kidney disease. Within the glomerulus, podocytes--differentiated epithelial cells crucial for filtration--are thought to lack substantial capacity for regeneration. Here we show that podocytes rapidly lose differentiation markers and enter the cell cycle in adult mice in which the telomerase protein component TERT is conditionally expressed. Transgenic TERT expression in mice induces marked upregulation of Wnt signaling and disrupts glomerular structure, resulting in a collapsing glomerulopathy resembling those in human disease, including HIV-associated nephropathy (HIVAN). Human and mouse HIVAN kidneys show increased expression of TERT and activation of Wnt signaling, indicating that these are general features of collapsing glomerulopathies. Silencing transgenic TERT expression or inhibiting Wnt signaling through systemic expression of the Wnt inhibitor Dkk1 in either TERT transgenic mice or in a mouse model of HIVAN results in marked normalization of podocytes, including rapid cell-cycle exit, re-expression of differentiation markers and improved filtration barrier function. These data reveal an unexpected capacity of podocytes to reversibly enter the cell cycle, suggest that podocyte renewal may contribute to glomerular homeostasis and implicate the telomerase and Wnt-β-catenin pathways in podocyte proliferation and disease.

Oct4 was a novel target of Wnt signaling pathway

The specific expression of Oct4 during early mouse development is required for the correct maintenance of pluripotent cells, and the regulatory control of the Oct4 expression is important. Wnt signaling could have multiple and/or complex effects on embryonic stem (ES) cells characteristics. Elucidation of the molecular mechanisms affecting Wnt signaling in ES cells could provide a better understanding of how these effects occur. The purpose of this study was to determine whether Oct4 was regulated by Wnt signaling in undifferentiated ES cells. Here, we report Oct4 as a novel target of β-catenin-mediated transcription. First, we observe that Wnt signaling pathway is activated in undifferentiated mouse ES cells. In 239T cells, Oct4 promoter was regulated by β-catenin. Through promoter mapping and chromatin immuno-precipitation assays, we found that Oct4 is a direct target of β-catenin/TCF-mediated transcription and the binding site at -875/-881 of Oct4 promoter is critical for b-catenin/TCF-dependent expression regulation. We further detect the expression of Oct4 in treatment with glycogen syntheses kinase (GSK)-3-specific inhibitor in mouse ES cells and HepG2 cells. We found that GSK-3-specific inhibitor can maintain the expression of Oct4 in ES cells and can enhance the expression of Oct4 in HepG2 cells. Our results suggest that Oct4 might be a novel target of β-catenin/TCF-mediated downstream gene in Wnt-activated cells.

Structural and functional studies of LRP6 ectodomain reveal a platform for Wnt signaling

LDL-receptor-related protein 6 (LRP6), alongside Frizzled receptors, transduces Wnt signaling across the plasma membrane. The LRP6 ectodomain comprises four tandem β-propeller-EGF-like domain (PE) pairs that harbor binding sites for Wnt morphogens and their antagonists including Dickkopf 1 (Dkk1). To understand how these multiple interactions are integrated, we combined crystallographic analysis of the third and fourth PE pairs with electron microscopy (EM) to determine the complete ectodomain structure. An extensive inter-pair interface, conserved for the first-to-second and third-to-fourth PE interactions, contributes to a compact platform-like architecture, which is disrupted by mutations implicated in developmental diseases. EM reconstruction of the LRP6 platform bound to chaperone Mesd exemplifies a binding mode spanning PE pairs. Cellular and binding assays identify overlapping Wnt3a- and Dkk1-binding surfaces on the third PE pair, consistent with steric competition, but also suggest a model in which the platform structure supports an interplay of ligands through multiple interaction sites.

Dickkopf1: a tumor suppressor or metastasis promoter?

Dickkopf1 (DKK1), a secreted inhibitor of the Wnt/β-catenin pathway, is a negative regulator of bone formation. DKK1 acts as a switch that transitions prostate cancer bone metastases from osteolytic to osteoblastic and also is an active indicator of poor outcome for multiple myeloma. However, in other tumor types, DKK1 upregulation or overexpression suppresses tumor growth. Thus, the role of DKK1 in cancer appears to be diverse. This raises a question: Could the increased levels of DKK1 still be tumor protective when observed in high levels in the serum of patients? Here, we summarize the diverse, seemingly contradicting roles of DKK1 and attempt to explain the apparent dichotomy in its activity. We propose that DKK1 is a critical secreted factor that modulates microenvironment. Based on the location and components of the microenvironment DKK1 will support different outcomes.

Clinicopathological and prognostic significance of serum and tissue Dickkopf-1 levels in human hepatocellular carcinoma

BACKGROUND

Although Dickkopf-1 (DKK1) is known to be a negative regulator of the Wnt/β-catenin pathway, it has been recently found to be upregulated in cancers.

AIMS

We investigated the clinical and prognostic significance of both serum and transcript DKK1 and its functional roles in human hepatocellular carcinoma (HCC).

METHODS

We evaluated the expression level of DKK1 in both tissue and serum samples from patients with HCC using GeneChip microarray and real-time-quantitative PCR and sandwich ELISA system respectively. The clinicopathological and prognostic significance of serum and tissue DKK1 levels was examined. Functional characterization of DKK1 with regard to cell migration, invasion and tumour growth was performed.

RESULTS

Both DKK1 transcript and serum protein were upregulated in a stepwise manner in human HCCs. Its transcript levels were associated with more aggressive tumour behaviour, in terms of venous invasion (P = 0.003), advanced tumour stage (P = 0.003). DKK1 transcript correlated with shorter overall (P = 0.006) and disease-free survival (P = 0.012), and higher serum DKK1 levels correlated with shorter disease-free survival (P = 0.046). Knockdown of DKK1 significantly reduced both migratory and invasive abilities of HCC cells, whereas overexpression of DKK1 enhanced the tumour formation efficiency and tumour growth in vivo.

CONCLUSIONS

Serum and tissue DKK1 levels increased in a stepwise manner in multistep hepatocarcinogenesis and had prognostic significance. DKK1 plays a functional role in cell migration, invasion and tumour growth.

The complex roles of Wnt antagonists in RCC

Renal cell carcinoma (RCC) is the most lethal of all the genitourinary cancers, as it is generally refractory to current treatment regimens, including chemotherapy and radiation therapy. Targeted therapies against critical signaling pathways associated with RCC pathogenesis, such as vascular endothelial growth factor, von Hippel-Lindau tumor suppressor and mammalian target of rapamycin, have shown limited efficacy so far. Thus, Wnt signaling, which is known to be intricately involved in the pathogenesis of RCC, has attracted much interest. Several Wnt signaling components have been examined in RCC, and, while studies suggest that Wnt signaling is constitutively active in RCC, the molecular mechanisms differ considerably from other human carcinomas. Increasing evidence indicates that secreted Wnt antagonists have important roles in RCC pathogenesis. Considering these vital roles, it has been postulated--and supported by experimental evidence--that the functional loss of Wnt antagonists, for example by promoter hypermethylation, can contribute to constitutive activation of the Wnt pathway, resulting in carcinogenesis through dysregulation of cell proliferation and differentiation. However, subsequent functional studies of these Wnt antagonists have demonstrated the inherent complexities underlying their role in RCC pathogenesis.

Structural basis of Wnt signaling inhibition by Dickkopf binding to LRP5/6

LDL receptor-related proteins 5 and 6 (LRP5/6) are coreceptors for Wnt growth factors, and also bind Dkk proteins, secreted inhibitors of Wnt signaling. The LRP5/6 ectodomain contains four β-propeller/EGF-like domain repeats. The first two repeats, LRP6(1-2), bind to several Wnt variants, whereas LRP6(3-4) binds other Wnts. We present the crystal structure of the Dkk1 C-terminal domain bound to LRP6(3-4), and show that the Dkk1 N-terminal domain binds to LRP6(1-2), demonstrating that a single Dkk1 molecule can bind to both portions of the LRP6 ectodomain and thereby inhibit different Wnts. Small-angle X-ray scattering analysis of LRP6(1-4) bound to a noninhibitory antibody fragment or to full-length Dkk1 shows that in both cases the ectodomain adopts a curved conformation that places the first three repeats at a similar height relative to the membrane. Thus, Wnts bound to either portion of the LRP6 ectodomain likely bear a similar spatial relationship to Frizzled coreceptors.

Heal thyself: using endogenous regeneration to repair bone

Bone has the capacity to repair itself after an injury, and this occurs in normal fracture repair. This reparative process can be harnessed to regenerate segments of bone using distraction osteogenesis, in which the healing bone is slowly stretched. The use of animal models is identifying the important sources of cells for this endogenous bone regeneration, signaling molecules that regulate this reparative process, and the environmental cues important for success bone regeneration. A more complete understanding of the cells and pathways involved in this process can be applied to improve the outcome of distraction osteogenesis and to the development of methods to enhance endogenous bone regeneration.

Dickkopf1--a new player in modelling the Wnt pathway

The Wnt signaling pathway transducing the stabilization of β-catenin is essential for metazoan embryo development and is misregulated in many diseases such as cancers. In recent years models have been proposed for the Wnt signaling pathway during the segmentation process in developing embryos. Many of these include negative feedback loops where Axin2 plays a key role. However, Axin2 null mice show no segmentation phenotype. We therefore propose a new model where the negative feedback involves Dkk1 rather than Axin2. We show that this model can exhibit the same type of oscillations as the previous models with Axin2 and as observed in experiments. We show that a spatial Wnt gradient can consistently convert this temporal periodicity into the spatial periodicity of somites, provided the oscillations in new cells arising in the presomitic mesoderm are synchronized with the oscillations of older cells. We further investigate the hypothesis that a change in the Wnt level in the tail bud during the later stages of somitogenesis can lengthen the time period of the oscillations and hence the size and separation of the later somites.

Crystal structures of the extracellular domain of LRP6 and its complex with DKK1

Low-density-lipoprotein (LDL) receptor-related proteins 5 and 6 (LRP5/6) are Wnt co-receptors essential for Wnt/β-catenin signaling. Dickkopf 1 (DKK1) inhibits Wnt signaling by interacting with the extracellular domains of LRP5/6 and is a drug target for multiple diseases. Here we present the crystal structures of a human LRP6-E3E4-DKK1 complex and the first and second halves of human LRP6's four propeller-epidermal growth factor (EGF) pairs (LRP6-E1E2 and LRP6-E3E4). Combined with EM analysis, these data demonstrate that LRP6-E1E2 and LRP6-E3E4 form two rigid structural blocks, with a short intervening hinge that restrains their relative orientation. The C-terminal domain of DKK1 (DKK1c) interacts with the top surface of the LRP6-E3 YWTD propeller and given their structural similarity, probably also that of the LRP6-E1 propeller, through conserved hydrophobic patches buttressed by a network of salt bridges and hydrogen bonds. Our work provides key insights for understanding LRP5/6 structure and the interaction of LRP5/6 with DKK, as well as for drug discovery.

HSulf-1 inhibits cell proliferation and invasion in human gastric cancer

The HSulf-1 gene encodes an extracellular 6-O-endosulfatase and regulates the sulfation status of heparan sulfate proteoglycans (HSPG). We have demonstrated that promoter hypermethylation is correlated with the HSulf-1 silencing in gastric cancer. To investigate the functional importance of HSulf-1 silencing in gastric cancer, we restored HSulf-1 expression in the gastric cancer cell line MKN28, which lacks endogenous HSulf-1. Following restoration of expression, HSulf-1 inhibited cell proliferation, motility, and invasion in vitro, as well as significantly suppressing the MKN28 xenograft model (P < 0.05). No noticeable changes in proliferation and motility were observed following restoration of HSulf-1 in another gastric cancer cell line, namely AGS cells. Interestingly, in MKN28 cells, which have been reported to be dependent on extracellular Wnt signaling, we found that HSulf-1 inhibited the transcriptional activity of the Wnt ⁄ β-catenin pathway and downregulated its targeted genes. Conversely, in AGS cells, in the constitutive Wnt ⁄ β-catenin pathway is active, HSulf-1 had no effect on the activity of the Wnt ⁄ β-catenin pathway. Furthermore, transfection of Wnt3a cDNA or β-catenin shRNA resulted in rescue or enhancement, respectively, of the effects of HSulf-1 in MKN28 cells. Furthermore, HSPG epitope analysis confirmed that HSulf-1 affected the structure of heparan sulfate on the cell surface. Together, the results of the present study suggest that extracellular HSulf-1 may function as a negative regulator of proliferation and invasion in gastric cancer by suppressing Wnt ⁄ β-catenin signaling at the cell surface.

Wnt antagonists bind through a short peptide to the first β-propeller domain of LRP5/6

The Wnt pathway inhibitors DKK1 and sclerostin (SOST) are important therapeutic targets in diseases involving bone loss or damage. It has been appreciated that Wnt coreceptors LRP5/6 are also important, as human missense mutations that result in bone overgrowth (bone mineral density, or BMD, mutations) cluster to the E1 propeller domain of LRP5. Here, we report a crystal structure of LRP6 E1 bound to an antibody, revealing that the E1 domain is a peptide recognition module. Remarkably, the consensus E1 binding sequence is a close match to a conserved tripeptide motif present in all Wnt inhibitors that bind LRP5/6. We show that this motif is important for DKK1 and SOST binding to LRP6 and for inhibitory function, providing a detailed structural explanation for the effect of the BMD mutations.